Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Photovoltaic (PV) devices or solar cells, which are a type of photosensitive optoelectronic device, are specifically used to generate electrical power. PV devices, which may generate electrical power from light sources other than sunlight, are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
Organic bulk heterojunction photovoltaic cells employ a blended combination of a p-type donor material and an n-type acceptor material (most commonly a fullerene derivative such as PC61BM or PC71BM or the analogous bis-indene adducts of C60 or C70 respectively). Charge separation is facilitated by migration of an exciton (formed by photoexcitation) to the heterojunction. Charge separation is promoted by the offset in levels of the HOMO of the p-type material and the LUMO of the electron acceptor. For donor materials blended with fullerene derivatives, the HOMO-LUMO gap should be in the range of 1.5 to 1.8 eV and the HOMO energy level should be low (−5.2 to −5.8 eV). The LUMO energy level of the donor should be 0.2 eV above the LUMO level of the acceptor to promote negative charge migration to the acceptor after photoexcitation. These concepts have been fully described in the publications by C. J. Brabec et al., Adv. Mater., 2010, 22, 3839; G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater. 2009, 21, 1323 (for donor polymers) and A. Mishra and P. Baeuerle, Angew. Chem. Int. Ed., 2012, 51, 2020 (for small molecules).
Bulk heterojunction solar cell energy conversion efficiencies in a single junction device have now been reported to reach ca.10% efficiency under standard AM1.5 conditions with 1 sun irradiation (100 mW cm−2). Efficiencies are determined by open circuit voltage Voc (ideally reaching up to or >1 V), short circuit current Jsc (ideally in excess of 13 mA cm−2) and fill factor FF (ideally in excess of 65%). Features that are known to contribute to improving these factors include (i) offset of the energy of the HOMO of the donor and the LUMO of the acceptor (allowing for a 0.3 eV energy difference to promote charge separation at the heterojunction); (ii) low HOMO-LUMO energy gap of the donor material to maximise photon absorption in the 800 nm region corresponding to the wavelength of maximum photon solar emission; (iii) ideal exciton diffusion length of about 10 nm which is largely determined by feature sizes and the morphology of the blend of donor and acceptor materials; (iv) balanced charge mobilities in the donor and acceptor materials; (v) high number average molar mass (for polymers) of the donor materials.
Recently it has become apparent that the combination of donor and acceptor type building blocks in well-defined polymers can lead to donor p-type materials having the preferred low HOMO-LUMO gap corresponding to a long wavelength absorption maximum ideal for solar energy. A useful donor building block is based on the benzodithiophene unit carrying “orthogonal” (for molecular stacking) thiophene substituents with long chain alkyl substituents (L. Huo, S. Zhang, X. Guo, F. Xu, Y. Li, J. Hou, Angew. Chem. Int. Ed., 2011, 50, 9697).
While polymeric materials comprising the combination of donor and acceptor type building blocks have been described, there remains the problem of providing donor and acceptor type building blocks having improved performance in photovoltaic devices.
Furthermore, while advances in polymeric materials continue to be made there remains a need to provide new polymers useful in the preparation of optoelectronic devices.